GROUP III. PTERIDOPHYTA. 347 



of four cells which are quadrants of a sphere, and this is followed 

 by the formation of a third wall (octant-icall), at right angles to 

 both the preceding, so that the embryo now consists of eight 

 cells which are octants of a sphere. In the Lycopodinse the 

 segmentation leading to the formation of quadrants and octants is 

 confined to the epibasal half, the hypobasal half remaining un- 

 divided or undergoing a few irregular divisions. From the 

 epibasal half, the primary stem and one or two primary leaves 

 (cotyledons) are developed in all cases. The hypobasal half gives 

 rise, in the Filicinse and Equisetinse, to the primary root and to 

 the foot, with but few exceptions (e.g. Salvinia in which there is 

 no primary root) : in the Lycopodinse the hypobasal half gives rise 

 to a filament consisting of a few cells, termed the suspensor 

 (compare Phanerogams). 



The foot (as also the suspensor) is an embryonic organ, no trace 

 of which persists in the adult. It is the organ of attachment of 

 the embryo-sporophyte to the gametophyte ; and it is also the 

 absorbent organ by which the embryo, until it is able to absorb and 

 assimilate food for itself, obtains its nourishment from the pro- 

 thallium (compare Bryophyta, p. 314). 



The development of a suspensor in the Lycopodinse is an adap- 

 tation correlated with the fact that the nourishment of the 

 embryo in that group depends upon its coming into direct contact 

 with the tissue of the massive gametophyte, the cells of which 

 are filled with nutritive substances. 



A primary root, that is, a root developed from the hypobasal 

 half of the oospore, and so situated at its origin that its growing- 

 point is in a straight line with that of the stem, only occurs 

 in the Filicinse and Equisetinse ; but even here it does not persist 

 as a tap-root in the adult : in these plants numerous adventitious 

 roots are developed. In the Lycopodinse, where there is no primary 

 root, all the roots are adventitious. 



Some adult forms are altogether without roots : as Salvinia, 

 and some species of Trichomanes, among Filicinse; Psilotum and 

 Tmesipteris, among Lycopodinse. The functions of the root are 

 discharged, in Salvinia by modified leaves, in the others by 

 modified branches. 



The branching of the root is generally lateral in the Filicinse 

 and Equisetinse; it is dichotomous in the Lycopodinse and in 

 Isoetes. In the former case, the lateral rootlets are developed, 

 in the Filicinse, from cells (rlnzogenic} of the endodermis which 



